Donald Trump has told Russia and Syria to "get ready" for a missile attack on the Assad regime, saying the bombs will be "nice and new and smart".

Mr Trump tweeted an extraordinary response to Russia's claim that it would shoot down any missiles fired at Syria following the chemical weapons attack on Douma.

The US President added: "You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"

Mr Trump described the relationship between the US and Russia as "worse now than it has ever been, and that includes the Cold War" but insisted there was "no reason for this".

Earlier, Alexander Zasypkin, the Russian envoy to Beirut, said: “If there is an American strike, then we... will shoot down the missiles and target the positions from where they were launched.

“In the past few days, we have seen an escalation towards a significant crisis."

Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and “smart!” You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!

Our relationship with Russia is worse now than it has ever been, and that includes the Cold War. There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race?

Mr Trump's tweets increase the pressure on Theresa May, the Prime Minister, to give her unequivocal backing to air strikes against targets in Syria, having said on Tuesday that she and Mr Trump would not "allow the use of chemical weapons to continue".

Russia responds to 'provocation'

On Wednesday, after Mr Trump's tweets, Moscow also suggested US plans to strike could be a pretext to destroy evidence of the alleged chemical weapons attack, which Russia has said was a staged "provocation" to justify Western intervention.

"Is the whole idea to quickly remove the traces of the provocation...(so) the international inspectors will have nothing to look for in terms of evidence?" asked foreign ministry spokeswoman Maria Zakharova in a Facebook post.

The Kremlin later added that it did not engage in "Twitter diplomacy", the Interfax news agency reported.

Kremlin spokesman Dmitry Peskov was quoted as saying that care should be taken not to aggravate the situation in Syria. "We do not participate in Twitter diplomacy," he was quoted as saying. "We support serious approaches. We continue to believe that it is important not to take steps that could harm an already fragile situation."

Syria described Mr Trump's threats as a "reckless escalation". "We are not surprised by such a reckless escalation from a regime like the United States which has fostered and continues to foster terrorism in Syria," an official source at the foreign ministry said.

Earlier in the day Eurocontrol, the pan-European air traffic control agency, warned civilian airliners to be cautious above the eastern Mediterranean because of the possibility of strikes in the next 72 hours.

The agency warned that both missiles launched from aircraft or from naval forces might be launched before the end of the week.

"Due to the possible launch of air strikes into Syria with air-to-ground and/or cruise missiles within the next 72 hours, and the possibility of intermittent disruption of radio navigation equipment, due consideration needs to be taken when planning flight operations in the Eastern Mediterranean,” the agency said in a statement.

Evidence mounts of chemical attack

The World Health Organisation said that 500 people were treated in last weekend’s suspected chemical weapons attack in Douma and demanded access to the site.

The global health agency said it estimated 500 were brought to hospital with "signs and symptoms consistent with exposure to toxic chemicals”.

"WHO demands immediate unhindered access to the area to provide care to those affected, to assess the health impacts, and to deliver a comprehensive public health response," said Peter Salama, the deputy director-general for emergency preparedness and response.

The Syrian regime has so far prevented UN agencies from visiting the site of the alleged attack, although Russian military inspectors have been allowed access.

Syria 'emptying' out military bases

As Mr Trump made his public threats on Twitter, Syrian opposition activists reported that regime forces and their allies were emptying bases and moving military equipment in an effort to shield it from expected US strikes.

Opposition groups said that Syrian regime was shifting military vehicles away from its airbase in Hama, a potential target for American cruise missiles.

Activists also said that Hizbollah, the Lebanese militant group supporting the Assad regime, was emptying out its military base near the T4 airbase in central Syria. Israeli jets reportedly struck the T4 base on Sunday night, killing 14 people, including seven Iranians.

Michael Horowitz, a senior analyst at the Le Beck geopolitical consultancy, said that the regime would probably move its most sensitive equipment close to Russian forces, in the hope that the US would be less likely to hit it and risk accidentally striking Russian troops.

“The Syrian military has already had time to take some contingency measures and evacuate some of the potential targets Washington could decide to strike, which could limit the impact of any possible American intervention,” he said.

“The Syrian air force in particular will likely redeploy to Russian or Russian-protected air base, in a bid to limit its exposure.”

Syrian regime aircraft could be moved to Khmeimim, the main Russian base in northwest Syria, or else clustered around known Russian positions inside of Syrian regime bases.

Syrian regime aircraft were largely absent from the skies of Syria on Wednesday, with only Russian warplanes in the air. Mr Horowitz said that was an indication of a change in behaviour by regime forces in anticipation of US strikes.

May gives backing to Trump

Meanwhile, Theresa May has given her strongest signal yet that Britain would support President Donald Trump in military action against the Syrian regime as the two leaders resolved “not to allow the use of chemical weapons to continue”.

Douma chemical attack

The Prime Minister spoke to both Mr Trump and the French President Emmanuel Macron by telephone during which all three agreed that Syrian President Bashar al-Assad had shown “total disregard” for international laws against the use of such weapons.

A Trump official upped the diplomatic tension by describing the chemical attack on Douma, Syria, as “genocide” and saying a military response was “appropriate”.

Mr Macron said the three countries would decide “within days” how to respond and discussed the possibility of hitting Syria’s “chemical capacities”. Sarah Huckabee Sanders, the White House press secretary, said: “All options are on the table.”

Russia wields UN veto

It came as Russia used its veto power at the UN Security Council on Tuesday evening against a US resolution to create new expert body to determine responsibility for Syria chemical weapons attacks, a move expected to increase the likelihood of US military intervention.

Russia urged the US to avoid taking military action, warning Washington that it will "bear responsibility" for any "illegal military adventure" it carries out.

Russian UN Ambassador Vassily Nebenzia said Washington's decision to put forward its resolution could be a prelude to a Western strike on Syria.

"I would once again ask you, once again beseech you, to refrain from the plans that you’re currently developing for Syria," he said after the council failed to approve a third draft resolution on chemical weapons attacks in Syria.

President's options | What could military action in Syria look like?

He added: "If you took the decision to carry out an illegal military adventure, and we do hope that you will come to your senses, well then you will have to bear responsibility for it.”

Whitehall sources suggest Mrs May would prefer to have the backing of Parliament in any decision to join a military response against Syria, but with both Mr Trump and Mr Macron eager to strike swiftly, that option is unlikely to be open to the Prime Minister.

Syria chemical attack | Read more

The Telegraph has learnt that no plans have been put in place to recall MPs before Monday, when they will return after the Easter recess, suggesting Mrs May has resigned herself to taking the decision in conjunction with her Cabinet, rather than seeking the support of the Commons.

Downing Street issued a more cautious statement than the White House, in which Number 10 said that the chemical attack “if confirmed” would represent fresh evidence of Assad’s “appalling cruelty”.